Heretofore four different techniques have been proposed for increasing the framework Si/Al ratio of crystalline zeolites by the extraction of aluminum atoms and at least the partial substitution of silicon atoms into the sites previously occupied by the extracted aluminum atoms. Numerous chemical extraction procedures using mineral acids, chelating agents such as ethylene diaminetetraacetic acid (H.sub.4 EDTA), solutions of CrCl.sub.3, gaseous fluorine, phosgene and the like are also known, but the vacancies left behind by the extracted aluminum atoms are not reoccupied by silicon atoms. In the case of the CrCl.sub.3 treatment, however, the substitution of chromium atoms into the vacated sites has been reported. In this regard see U.S. Pat. No. 3,937,791, Garwood et al.
Of the prior reported silicon insertion procedures, the earliest and the most thoroughly investigated involves the steaming of a hydrogen or ammonium exchanged form of the starting zeolite at temperatures usually in excess of 550.degree. C. using a steam environment containing at least 2 psia water vapor pressure. For example, U.S. Pat No. 3,591,488 discloses that the hydrogen or ammonium form of a zeolite may be treated with steam at a temperature ranging from about 800.degree. F. to about 1500.degree. F. (about 427.degree. C. to about 816.degree. C.), and thereafter cation-exchanged with rare earth cations.
It appears to be generally accepted among those skilled in the art that in the steaming treatment aluminum atoms extracted from the crystal lattice and moved to the interstitial space are immediately replaced by a "nest" of four hydroxyl groups, and that some portion of these hydroxyl nests are in turn replaced by silicon atoms. There is not general agreement as to the source of the replacing atoms, i.e., pre-existing framework silicon atoms or occluded silicon-containing impurities, or the degree to which "healing" of the lattice occurs by virtue of silicon insertion. It has been shown, however, that steamed faujasite type zeolites develop a secondary mesopore system with pore radii in the range of 15 to 19 Angstroms, an indication that the silicon substitution mechanism does involve the migration of framework silicon atoms and possibly the elimination of entire sodalite cages. See in this regard, U. Lohse et al, Z. Anorg. Allg. Chem., 1980, 460, 179.
The second type of silicon insertion procedure heretofore proposed is reported in U.S. Pat. No. 4,503,023. In that patent specification Skeels et al describe a process for dealuminating a zeolite by treatment of the zeolite with a fluorosilicate salt in an amount of at least 0.0075 moles per 100 grams of the zeolite (on an anhydrous basis), the fluorosilicate salt being provided in the form of an aqueous solution having a pH in the range of 3 to about 7. The aqueous solution of the fluorsilicate salt is brought into contact with the zeolite at rate sufficiently slow to preserve at least 80 percent, preferably at least 90 percent, of the crystallinity of the starting zeolite. The fluorosilicate extracts aluminum from the zeolite lattice framework and substitutes silicon therein, thus increasing the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of the zeolite without introducing large numbers of defect sites into the framework. The products of this process resulting from the treatment of zeolite Y are known in the art as LZ-210.
A third procedure alleged to accomplish framework insertion of silicon is reported in Beyer et al in "Catalysis by Zeolites," ed. B. Imelik et al (Elsevier, Amsterdam, 1980) p. 203 et seq. In this procedure, apparently operable only in the case of zeolites having the faujasite type of structure, the starting zeolite is contacted with silicon tetrachloride vapor in an inert atmosphere of nitrogen at elevated temperatures, typically about 400.degree. C. to 500.degree. C. The reaction which occurs is ideally EQU Na.sub.x (AlO.sub.2).sub.2 (SiO.sub.2).sub.y +SiCl.sub.4 .fwdarw.Na.sub.x-1 (AlO.sub.2).sub.x-1 (SiO.sub.2).sub.y +AlCl.sub.3 +NaCl
It can be theorized, however, that this reaction mechanism occurs only in the total absence of water and proceeds uncontrollably to the formation of an amorphous product. When the starting zeolite is not fully dehydrated, however, the SiCl.sub.4 reagent can react with the residual water to form HCl which in turn attacks framework aluminum atoms and causes dealumination. Stabilization then proceeds in much the same manner as in the case of steam stabilization except that the stabilizing agent is SiCl.sub.4 rather than H.sub.2 O. In any event, the Beyer et al products have a relatively low ratio of total aluminum to non-aluminum cations which is not characteristic of the LZ-210 products of the aforementioned Skeels et al process.
The fourth method proposed for framework silicon insertion utilizes silicon initially present in another portion of the crystal lattice rather than from an extraneous source. The process is reported by G. W. Skeels in U.S. Pat. No. 5,100,644 and comprises contacting and reacting a zeolite starting material with an aqueous solution of a bifluoride salt in proportions such that there is from about 0.5 to 10 moles of bifluoride ion per mole of zeolite framework aluminum the contact between zeolite and bifluoride salt solution being carried out at a temperature of from about 60.degree. C. to 100.degree. C. and the starting pH of the bifluoride salt solution being not greater than 7. Framework silicon and aluminum atoms are removed from the zeolite and at least some of the removed silicon atoms are inserted into sites vacated by removed aluminum atoms.